Viruses, blue mold hit tobacco hard Early blue mold infestations, a resurgence of tobacco mosaic virus, increasing incidences of tomato spotted wilt virus and changing races of black shank have kept North Carolina flue-cured tobacco growers on their toes this summer.

Participants in this year's North Carolina Flue-Cured Tobacco Tour saw how growers and specialists are addressing these and other challenges.

The tobacco mosaic virus resurgence took farmers and specialists somewhat by surprise. Since the disease has been around for a long time, it has been thoroughly researched and most growers thought they knew how to keep it from spreading. They got some surprises this year.

"We've seen pretty much a slight but steady decline in damage from tobacco mosaic virus over the last several years," says plant pathologist Tom Melton.

"A few growers had more problems last year. We saw a significant resurgence this year. Two or three weeks after transplanting, a lot of growers saw 50 to 100 percent of their plants infected. In virtually all of those cases, the plants were infected when they came out of the greenhouses."

It might seem obvious that someone on these farms was not following proper sanitary procedures. Most growers and farm laborers know they must not smoke or handle other tobacco products around tobacco transplants, in greenhouses or in the field. They have learned to wash their hands in milk or take other precautions to prevent the spread of this virus.

"If it had been 20 farmers in North Carolina who had the problem, I would believe that someone got sloppy," Melton notes.

"But when it is this widespread, from Florida through Virginia, that makes me feel like something about the virus has changed. We're looking at all the possibilities."

Some growers speculate the virus is being transmitted in tobacco seed. Seedsmen insist that their seed have been treated to prevent such an occurrence and that the virus is not transmitted in seed. Melton and specialists in other states are collecting samples of seed from farmers who experienced significant tobacco mosaic virus problems. They are planting the seed and monitoring to see if the disease develops.

Other growers say they have seen more problems in specific tobacco varieties like NC 71 and NC 72.

Melton is also conducting tests to see if the virus can be transmitted through transplant or irrigation water or even through insects. Previous tests have shown that this is not an insect transmitted virus. But, if the virus has changed, it is possible that insects are now introducing the virus to tobacco plants.

While the disease has been widespread and in some cases has infected entire fields, Melton says the generally good growing season has allowed most plants to overcome the disease. Some 100 percent infected fields will lose from five to 40 percent of their value to tobacco mosaic virus in 2000.

Tomato spotted wilt virus, a disease that has caused tremendous losses in Georgia, is also moving into the upper Southeast. Mark Keene, Extension agent in Onslow County says the disease is causing a lot of concern among North Carolina tobacco farmers.

"We have seen a lot of tomato spotted wilt virus in several fields," he says. "The good news is that the tests Melton has done with Actigard look real good."

Actigard should be labeled for blue mold control in tobacco before the summer is over. However, it will not be immediately labeled for tomato spotted wilt virus control.

"Tomato spotted wilt virus is a disease we've said there was no control for," Melton says. "Actigard is doing a good job of knocking the disease down. We had 28 percent of the plants in our control plots infected. Treating with Actigard knocked that down to 7.3 percent. But, for this product to be effective, we need to start spraying in the greenhouse and continue to spray on a 10 to 13 day schedule in the field.

"This is an intensive spray program that is totally preventive. The label we are expecting for Actigard is for blue mold control and does not allow applications until tobacco is 18 inches tall or taller. We have seen erratic phytotoxicity where we have sprayed smaller plants. Actigard is a good blue mold material."

Blue mold got an early start in eastern North Carolina and has continued to spread across the state. Because the humidity in tobacco fields has remained high through most of the growing season, the disease has remained a threat all season.

"The main reason we have not had more losses to blue mold this year is most growers put on preventive fungicides when they heard from the blue mold advisory that the fungus was on the way," Melton says.

"This has been a good year for growers to learn that hot weather alone will not stop blue mold. Dry weather will slow it down."

Growers also learned that it is a lot easier to control blue mold in greenhouses than when it gets into the fields. Most of this year's problems began in greenhouses. Next year Melton will recommend that growers begin applying preventive fungicide sprays in greenhouses at least by the time they begin clipping. This recommendation is similar to the recommendations for blue mold prevention growers formerly followed in field plantbeds.

The only fungicide labeled for greenhouse use is Dithane DF. Growers can use Dithane DF in the field, but Melton generally recommends Acrobat MZ.

"Ridomil is still available for field use as a preventive," Melton says.

"Over the last few years the disease has not responded as well to Ridomil because the blue mold pathogen has changed. There are other fungicides coming on, but none are as effective as a preventive as Ridomil was. Growers should change their thinking on blue mold control from using something they can apply to the soil and not worry to following the blue mold forecasting or advisory program and then applying something like Acrobat MZ when it is called for."

Because of the good growing season, black shank has caused very little obvious crop damage in North Carolina this season. But graduate student Melinda Sullivan says problems are brewing.

Sullivan is studying how different races of black shank affect various flue-cured varieties. There are two strains of the black shank pathogen, Race 0 and Race 1, which are attacking the crop. Sullivan's work shows that continuous or frequent planting of specific tobacco varieties can cause a shift in black shank races, making control more difficult.

"Based on Melinda's work, tobacco growers may have to start managing tobacco varieties like soybean growers manage varieties," says Melton. "Her work also makes it clear that rotation pays."

Sullivan planted K 326 with low resistance to both races of black shank, K 346 with high resistance to both races, and NC 71 with high resistance to Race 0 and low resistance to Race 1 in a field with high levels of both Race 0 and Race 1 black shank. She has both long-term rotations and continuous cropping.

"The most striking thing we have seen is when we plant NC 71, we see an increase in Race 1 black shank," she says. "When we plant NC 71 for two years consecutively, we get an increased incidence of black shank. Continuous planting of a variety that has resistance to only one race of black shank allows the other race to build up. Over time that variety effectively loses its resistance in that field."